PSN-L Email List Message

Subject: Re: seismometer performance
From: Brett Nordgren Brett3mr@.............
Date: Sat, 16 Feb 2008 20:24:02 -0500

Hi Chris,

If you're interested in Ni-Span-C you might find this of interest.

I think you can get if from Special Metals Corp.  I see that they have some 
kind of operation in the UK, so it might be worth a call.  I don't know how 
much you can specify regarding the heat treatment, especially in 
sample/prototype quantities.  It's likely that you would have to find 
someone in the area who does that and have them do the heat treating to 
your specs.  It is not quite as strong as spring steel, so you'd need to 
take that into consideration when designing the spring to avoid having it 
take a set (as well as doing all sorts of other interesting nonlinear things).

It's obvious that constructing a low T.C. spring is not for the faint of heart.

Regarding temperature control, you might be able to control the entire 
device fairly well if it's not too large.  I had an oven circuit in one of 
our power supplies that would control to about 1/1000 deg C, though the 
thermal design of the oven meant that the actual regulation was a good deal 
poorer.  I'd imagine you could control a small copper box to a fraction of 
a degree.  That's one good reason for designing small.


At 05:28 PM 2/16/2008 -0500, you wrote:
>In a message dated 2008/02/16, Brett3mr@............. writes:
>>Yes, I agree the forcing coil/magnet also introduce effects similar to what
>>I was describing for the integrator capacitor; except that you can avoid
>>the coil resistance issues by using a current driver.  It's obvious that
>>you can't depend on electronic feedback to do all your centering, forever,
>>but must occasionally turn a screw to unload the feedback loop.
>Hi Brett,
>        My train of thought was to either compensate or to drastically 
> reduce the main error drifts. I use a small thermostat circuit to keep 
> crystal temperatures constant to better than 0.1 C Deg. I could wrap one 
> nicely around a capacitor. If you used an electromagnet, you could keep 
> the field constant to a few ppm. An alternative would be to control the 
> temperature of the magnets.
>        The biggest drift is likely that of a steel spring. Can you get 
> Ni-SpanC wire? sheet? I can get Invar wire, but not Elinvar at the moment.
>        Regards,
>        Chris Chapman

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